Shearographic imaging machine

ABSTRACT

The invention relates to an apparatus for performing electronic shearography on a test object, especially a tire or retread tire. The apparatus uses a laser light source to illuminate the test object. An optical element through which electromagnetic radiation is reflected from the test object is transmitted and forms a random interference image. The random interference image is electronically processed to provide a video animation of the effects of stress on the test object.

CROSS-REFERENCE TO RELATED PATENT APPLICATION

[0001] This patent application is a continuation of copending U.S.patent application Ser. No. 09/334,311 filed Jun. 16, 1999 thedisclosure of which is hereby incorporated by reference.

FIELD OF THE INVENTION

[0002] The present invention relates generally to the field ofnondestructive testing. Specifically, the present invention relates tothe technique of electronic shearography. More specifically, the presentinvention relates to the use of electronic shearography to detectdefects in vehicle tires by animating shearograms produced while thetires undergo a varying stress continuum.

BACKGROUND OF THE INVENTION

[0003] The technique of shearing interferometry, or shearographyinvolves the interference of two laterally displaced images of the sameobject to form an interference image. Conventional shearographic methodsrequire that a first interference image (or baseline image) be takenwhile the object is in an unstressed or first stressed condition, andanother interference image be taken while the object is in a secondstressed condition. Comparison of these two interference images(preferably by methods of image subtraction) reveals information aboutthe strain concentrations and hence the integrity of the object in asingle image called a shearogram. In particular, shearography has beenshown to be useful to detect strain concentrations and hence defects invehicle tires, especially retread vehicle tires.

[0004] In conventional electronic shearography, interference images arestored in a computer memory and are compared electronically to producesingle static shearograms. Because all the data are processedelectronically, the results of the analysis can be viewed in “realtime”. “Real time”, as used in the prior art, refers to the ability toview the shearogram nearly instantaneously after the second interferenceimage has been taken.

[0005] An apparatus and method for performing electronic shearography isdescribed in U.S. Pat. No. 4,887,899 issued to Hung. The apparatusdescribed in the cited patent produces an interference image by passinglight, reflected from the test object, through a birefringent materialand a polarizer. The birefringent material, which can be a calcitecrystal splits a light ray, reflected from the object, into two rays,and the polarizer makes it possible for light rays reflected from a pairof points to interfere with each other. Thus, each point on the objectgenerates two rays, and the result is an interference image formed bythe optical interference of two laterally displaced images of the sameobject.

[0006] Prior to the developments disclosed in the Hung patent, thespatial frequency of the interference image produced in shearographicanalysis was relatively high requiring the use of high resolutionphotographic film to record a useful interference image. The developmentdisclosed in the Hung patent produces an interference image with arelatively low spatial frequency because the effective angles betweenthe interfering rays are small. Therefore, the interference images canbe recorded by a video camera, a video camera normally having much lessresolving capability than a high density or high resolution photographicfilm. By storing an interference image of the object in its initial,unstressed condition, and by comparing that interference image,virtually instantaneously, by computer with another interference imagetaken under a different level of stress, a “real time” image orshearogram of the resultant strains on the object can be observed. Eachpoint on the actual interference image is generated by the interferenceof light emanating from a pair of distinct points on the object.Therefore, each pixel of the video camera is illuminated by lightreflected from those two points. If the overall illumination remainsconstant, then any variations in the pixel intensity, in theinterference image, will be due only to changes in the phaserelationship of the two points of light.

[0007] When the initial video image of the interference image is stored,an initial intensity for each pixel is recorded, as described above. Ifdifferential deformations occur in the object, such deformations willcause changes in the subsequent interference image. In particular, theintensity of a given pixel will change according to change in the phaserelationship between the two rays of light, reflected from the twopoints on the object, which illuminate the pixel. The phase differencescan be either positive changes, causing the pixel to become brighter ornegative changes, causing the pixel to become darker. Whether the pixelbecomes brighter or darker depends on the initial phase relationship andthe direction of the change of phase. Due to the cyclic nature of phaseinterferences, as the deformation of the object continually increases,the intensity at a given pixel may pass through a complete cycle. Thatis, the intensity of the pixel might increase to a maximum (positive)difference, then return to the original intensity, and then continue toa maximum (negative) difference, and so on.

[0008] In systems of the prior art, a single shearogram is derived fromtwo single static interference images taken at two distinct stresslevels. The single shearogram is then viewed by an operator for analysisif multiple shearograms are taken, the analysis is done one shearogramat a time. Thus, the operator attendance time, required to perform athorough stress analysis, is substantial. Further, a single shearogrammay falsely show light features that appear to be defects (referred toas “false positives”). These “false positives” are caused by differentreflective characteristics on the surface of the test object and appearas defects when a static shearogram is viewed. Further still, in astatic shearogram some real defects may be “washed out” and thus notvisible (referred to as “false negatives”), at certain (particularlyhigh) stress levels. These “washed out” effects are caused byshearographic fringe lines that are not spatially separated enough to bevisibly distinguishable and therefore appear to be aberrational lighteffects rather than real defects in the test object. Thus, a singlestatic shearogram may contain inaccurate information with regards to thedefects actually present. Furthermore, an operator having to analyze alarge number of shearograms requires a large amount of operatorattendance time.

[0009] There is a need and desire for an improved method of presentationof shearographic images that provide advantages over the prior art.There is also a need and desire for a method of presenting shearographicimages that provide improved accuracy, shorter attendance times by anoperator, and shorter overall cycle times for a test object. Further,there is a need and desire for a method of presenting shearographicimages that reduce the undesirable effects of false negatives bypreventing “wash out” of larger defects at high stress levels. Furtherstill, there is a need and desire for a method of presentingshearographic images that allows real defects to be distinguished overlight features that otherwise may be confused as defects, therebyminimizing false positives.

SUMMARY OF THE INVENTION

[0010] The present invention relates to an apparatus for performingelectronic shearography on a test object. The apparatus includes asource of coherent electromagnetic radiation for illuminating the testobject, and an optical element through which electromagnetic radiationreflected from the test object is transmitted forming an interferenceimage. A detector converts the interference image into an electricalsignal representative of the interference image. An animation device iscoupled to the detector. The animation device receives the electricalsignal representative of the interference image. The animation deviceretains image information derived from the electrical signalsrepresentative of the interference image at a predetermined frame rate.The animation device compares the retained interference imageinformation with a baseline interference image to produce a shearogramimage, and the animation device is adapted to play a series ofsequential shearogram images. A display device is coupled to theanimation device, providing visualization of the sequential shearogramimages.

[0011] The present invention further relates to a method of analyzing atest object. The method includes directing coherent electromagneticradiation onto a test object, providing electromagnetic radiationreflected from the test object to an optical shearing device, theoptical shearing device creating an interference image, and directingthe interference image, emerging from the shearing device, onto adetector. The method further includes capturing an electrical signal,communicated from the detector, in a capture device, the electricalsignal being representative of the interference image, storinginterference image information in a memory device communicated from thecapture device and comparing interference image information stored inthe memory device, to a stored interference image to produce ashearogram image. The method still further includes repeating theaforementioned steps at varying stress levels and displaying shearogramimage information at a frame rate.

[0012] The present invention still further relates to an apparatus forperforming electronic shearography on a tire undergoing varying statesof stress. The apparatus includes a source of coherent electromagneticradiation for illuminating the tire, a birefringent material throughwhich electromagnetic radiation reflected from the tire is transmitted,and a polarizer through which electromagnetic radiation, emerging fromthe birefringent material, is transmitted, the birefringent material andthe polarizer cooperating to form an interference image. The apparatusalso includes a video camera, the video camera converting theinterference image to an electrical signal and a video capture circuitcoupled to the video camera, the capture circuit receiving theelectrical signal from the camera, the electrical signal beingrepresentative of the interference image, the capture circuit retainingimage information derived from the electrical signals representative ofthe interference image at a frame rate. Further, the apparatus includesa computer coupled to the capture circuit, the computer adapted tocompare sequential interference images retained by the capture circuitto a baseline image to produce a shearogram image, the computer adaptedto play the sequential shearogram images, and the computer including adisplay device coupled to the computer providing visualization of thesequential shearogram images and a memory device, coupled to thecomputer, the memory device being adapted to store the interferenceimage information retained by the capture circuit.

BRIEF DESCRIPTION OF THE DRAWINGS

[0013] The invention will hereafter be described with reference to theaccompanying drawings, wherein like reference numerals denote likeelements in the various drawings, and;

[0014]FIG. 1 is a schematic block diagram of a shearographic imagingsystem;

[0015]FIG. 2 is a schematic diagram of a shearographic imaging systemshowing a cross-section of a tire as the test object;

[0016]FIG. 3 is a schematic diagram of a shearographic camera at twodifferent orientations relative to the tire; and

[0017]FIG. 4 is a graphical representation of the deformation of a testobject, showing the corresponding shearographic fringe pattern produced.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0018] The present invention utilizes basic concepts of electronicshearography. More details of electronic shearography are given in U.S.Pat. No. 4,887,899, the disclosure of which is incorporated by referenceherein.

[0019] Referring now to FIG. 1, a schematic block diagram of anarrangement for practicing electronic shearography is depicted. Coherentelectromagnetic radiation or coherent light is produced by a laser 10,the laser light being directed through a fiberoptic cable 15 (oralternatively directed by a mirror or a set of mirrors or provideddirectly) to a beam expander or illuminator 20. Beam expander 20 directsthe coherent light onto a test object 25. The surface of test object 25is illuminated and reflects light into a shearography camera 30.Shearography camera 30 includes an optical element 35, a lens 40 forfocusing the light, and a detector 45. Optical element 35 may be abirefringent material and a polarizer, the birefringent material being acalcite material such as a Wallestein prism. The optical element ishowever not limited to a birefringent material and a polarizer, otherelements such as a defraction grating, a Mickelson mirror, or anappropriate wave plate may be applied. Further, optical element 35 maycontain other optics, such as, but not limited to a quarter-wave plate.Detector 45 may be a traditional video camera, a digital video camera, acharge coupled device (CCD), or other photo sensitive detectionequipment.

[0020] The output of detector 45 is coupled to an animation device suchas a computer 50. Computer 50 includes a video capture circuit 55, acentral processing unit 60, and a memory 65. Alternatively, computer 50may include a logical extractor that is configured to extractshearographic images from memory in a predetermined manner. The logicalextractor may be embodied in hardware or alternatively in softwarewithin computer 50. Video capture circuit 55 may be a dedicated videocard or a frame grabber preferably capable of capturing entire videoimages at a rate of at least 15 frames per second. However, videocapture circuit 55 may be capable of capturing video images at anysuitable rate. Central processing unit 60 may be any of a number ofconventional microprocessors or a dedicated microprocessor device.Detector 45 is coupled to central processing unit 60, central processingunit 60 being coupled to video capture circuit 55 and memory device 65.Central processing unit 60 is further coupled to a display unit 70,which may be a CRT (cathode ray tube) display, an LCD (liquid crystaldisplay), or the like.

[0021] In operation, coherent light emanating from beam expander 20 isreflected from test object 25. Optical element 35 collects the reflectedlight from object 25 causing an interference image to be created. Theinterference image is focused on detector 45 through lens 40.Conventionally, a first interference image is taken while test object 25is in a first stressed condition, and a second interference image istaken with object 25 in a second stressed condition. The twointerference images are then compared by a process of subtracting oneimage from the other and the shearogram is created and displayed on amonitor.

[0022] In the present invention, test object 25 undergoes a sequence ofor continuum of varying stress levels. Detector 45 continuously capturesthe interference image from optical element 35 and communicates theinterference image to computer 50, during the stress cycle. Capturecircuit 55 electronically captures entire interference images at a rateof at least 15 frames per second. Capture circuit 55 communicates theinterference images to central processing unit 60. Central processingunit 60 compares the interference image to a baseline interference imageof the object in the unstressed or near unstressed state (oralternatively any chosen stress state), by a process of subtracting oneinterference image from the baseline interference image, thereby forminga shearogram. Each shearogram image is simultaneously displayed ondisplay unit 70 and stored in memory device 65. After the series ofvarying stress levels has been completed, microprocessor 60 (oralternatively a logical extractor) recalls the sequence of shearogramimages captured by capture circuit 55 and replays them in sequence ondisplay unit 70. The sequential display of these shearogram images, at arate of at least 15 frames per second, produces a shearographicanimation of the shearograms produced during or after stressing of testobject 25.

[0023] Test object 25 may be a relatively large object, such as a tire200, as depicted in FIG. 2. A shearographic camera 230 that is rotatablewithin the inside of the bead 202 of tire 200 is depicted in FIG. 2.(Alternatively, tire 200 may be rotated and camera 230 may bestationary.) Shearographic camera 230 includes a laser 235 producing acoherent beam of light to illuminate the inside of tire 200.Shearographic camera 235 is further coupled to a computer 240 having adisplay 245, computer 240 and display 245 being used for dataacquisition and animation of the resultant shearographic images.

[0024] When used for detection of defects in tires or retread tires,shearographic imaging camera 230 may be positioned inside the tiredepicted as position A in FIG. 3 or outside the tire as depicted in FIG.3 by position B. Having shearographic camera 230 in position A allowsfor detection of defects in the tread area of tire 200. Havingshearographic camera 230 in position B provides for examination of thebead area and side wall area of tire 200.

[0025] Referring back to FIG. 2, in operation, shearographic camera 230and tire 200 may be placed into a vacuum chamber capable of subjectingtire 200 to a vacuum producing stresses on tire 200 by producing apositive pressure (relative to the pressure inside the vacuum chamber)in voids within tire 200 causing a bulge 250. Referring to FIG. 4, thebulge may be caused by a defect 260, defect 260 possibly being but notlimited to a delamination between two layers of the tire or a void inthe molded material. When subjected to a vacuum, bulge 250 appearsbecause of positive pressure within the void space of bond 260. Thegraph of FIG. 4 depicts the slope of bulge 250 by line 270. The graph ofFIG. 4 further depicts a fringe pattern, including groups of rings 280and 290, produced by the differencing of two optical interference imagesproduced by shearographic camera 230. Fringe patterns 280 and 290 of ashearogram image is produced by computer 240 (by the method ofdifferencing or by any other image resolving technique) appear as a setof roughly concentric, substantially circular fringe lines correspondingto slope 270 of bulge 250. Fringe patterns 280 and 290 are a contourmapping of the absolute value of slope 270 of bulge 250. Therefore,because bulge 250 is substantially symmetric, fringe patterns 280 and290 appear to be mirror images of each other.

[0026] Referring back to FIG. 2, in operation, shearographic camera 230takes a series of interference images that are communicated to computer240 while tire 200 undergoes varying vacuum or stress cycle. In apreferred embodiment tire 200 undergoes a depressurization cycle andthen a pressurization cycle to return the tire to an unstressed state.Because the field of view of shearographic camera 230 is limited by thefield of view of the optical elements and by the size of the tire, atire must be sectioned into a number of sectors ranging from four totwelve, or more. In an exemplary embodiment, tire 200 is sectioned intonine different sectors. Shearographic camera 230 therefore views an areacorresponding to 40° of arc of tire 200. After the depressurization andpressurization cycle, camera 230 is rotated to the next sector, therethe depressurization and pressurization cycle is repeated. Computer 240continues to collect data and may, in a preferred embodiment,simultaneously display data on display 245 throughout the entirety ofthe nine sector cycle. The shearograms are generated and displayed at arate such that they appear to be animated.

[0027] Referring now to FIG. 5, a display 300 is depicted, the displaybeing divided into nine different sectors, each sector 310 correspondingto an approximate 40° arc of the inside of a tire. Alternatively,however, each sector 310 could correspond to any specific field of view,of a tire, for a shearographic camera, such as shearographic camera 230.Computer 240 as depicted in FIG. 2, which may be connected to display300, is capable of displaying a plurality of animations simultaneouslyas depicted in FIG. 5. FIG. 5 depicts a static screen shot of a typicaldisplay, however, display 300 actually shows animations or sequentialimaging of shearogram images produced by computer 240 at a rateproviding an animated effect and in a preferred embodiment at a rate of30 frames per second. A display having multiple animation windows orscreen sectors provides the clear advantage that an operator may observethe animations simultaneously looking for the appearance of indicationsof deformations due to defects. This simultaneous observation permitsless attendance time by an operator, therefore providing substantialtime savings without substantial loss of accuracy. Capturing andproviding animation preferably at 30 frames per second (or alternativelyany suitable animation rate) provides animations that are sufficientlysmooth to be useful to an operator.

[0028] The advantages of animating the sequence of images is thatanimation improves accuracy in the detection of defects. Light effectsthat would appear as “false positives” in a static shearogram are notmanifested as defects when animated, due to the absence of apparentmotion induced by the animation. A fringe pattern caused by a realdefect will tend to “grow” or “shrink” and the intensity of fringe lineswill appear to cycle during the animation, due to the continuallychanging stress state on the test object. Furthermore, real defects thatmay be “washed out” in a static shearogram or even in an integration ofmultiple shearographic images, become apparent with animation of theshearographic images.

[0029] Animation of the shearographic images allows visualization ofdefects at a multiplicity of stress states, some of the stress statesmay not cause the “washed out” effect and further the apparent motioncreated by animation of the images manifests a real defect as opposed tothe light effect. Animation of the shearograms goes through asubstantial continuity of stress states, therefore defects that may notbe present at two chosen stress states become apparent in the animation.These advantages in animation of the shearographic images provide betteraccuracy in detecting defects and provides for shorter analysis times byan operator.

[0030] It has been recognized that a number of signal processingtechniques, such as, but not limited to the use of fuzzy logic, neuralnetworks, artificial intelligence, and pattern recognition techniques,may be applied to perform automatic defect identification. However,systems such as this tend to be inherently complex and substantiallycostly. Therefore, retaining a human operator, but cutting down on theoperators' required attendance time by providing the operator withnumerous simultaneous animations, has the effect of providingsubstantial cost savings.

[0031] Although animation of shearographic images may be preferable at arate of at least 15 frames per second, it should be noted that framerates of less than 15 frames per second may also be used effectively,however the animation may appear discretized as compared to an animationrunning at least 15 frames per second. Further, it should be appreciatedthat frame rates of more than 30 frames per second may be advantageousin specific applications and may become simpler to implement asmicroprocessor and video capture technology is improved.

[0032] It should be appreciated that although a differencing approach toproducing each shearogram is described above, the methods andapparatuses disclosed may be applied to different image resolvingtechniques, including but not limited to continuous integration.Continuous integration describes the process of taking a firstinterference image and subtracting a second interference image toproduce a first shearogram. A third interference image is taken andsubtracted from the first shearogram to produce a second shearogram. Afourth interference image is then taken and subtracted from the secondshearogram to produce a third shearogram. This sequence is continuedthroughout the testing cycle. The continuous integration technique andother techniques known to those of ordinary skill in the art, lendthemselves to the animation techniques disclosed above and can beapplied thereto without departing from the spirit and scope of thepresent invention.

[0033] The process and apparatus described above should be appreciatedto optimize a number of competing factors associated with shearographicimaging, especially as applied to the testing for defects in retreadtires (although clearly not limited to this application). Thesecompeting factors include, but are not limited to, maximizing data,maximizing accuracy, minimizing operator attendance time, availablelight wavelengths, object size, equipment costs, and optical field ofview. By animating shearograms in a plurality of sectors on a displayscreen, a number of these competing factors are optimized.

[0034] It is understood that, while the detailed drawings and examplesgiven describe preferred exemplary embodiments of the present, they arefor purposes of illustration only. The method and apparatus of theinvention is not limited to the precise details and conditionsdisclosed. For example, the invention is not limited to the specificframe rates at which shearographic images are captured or displayed.Further, the number of sectors of the test object is completely variableand, the object being tested may be any of a number of test objects.Still further, the method by which the test object is placed understress may be any of a number of techniques. Still further, otheroptical systems that produce interference images, other thanshearographic camera 30, may be applied to produce shearograms. Variouschanges may be made to the details disclosed without departing from thespirit of the invention, which is defined by the following claims.

What is claimed is:
 1. An apparatus for performing electronicshearography on a test object comprising: a shearography camera fortaking an interference image of the test object, an image processorcoupled to the shearography camera, the image processor being adapted toreceive a plurality of sequential interference images from theshearography camera, produce a plurality of sequential shearogram imagesof the test object from the interference images and animate thesequential shearogram images to represent dynamically changing stressstates on the tire, and a display coupled to the image processor forproviding visualization of the animation of the sequential shearogramimages.
 2. The apparatus according to claim 1 wherein the imageprocessor further includes a memory device for storing the shearogramimages.
 3. The apparatus according to claim 2 wherein the imageprocessor is adapted to animate the shearogram images stored in thememory device.
 4. The apparatus according to claim 1 wherein imageprocessor is adapted to receive interference images from theshearography camera at a frame rate of at least fifteen frames persecond.
 5. The apparatus according to claim 1 wherein the imageprocessor is adapted to animate the shearogram images at an animationrate of at least fifteen frames per second.
 6. The apparatus accordingto claim 1 wherein the image processor is adapted to substantiallysimultaneously animate multiple shearogram image sequencesrepresentative of different sections of the test object.
 7. Theapparatus according to claim 1 wherein the image processor is acomputer.
 8. A method for analyzing a test object comprising: (a) takingan interference image of a test object, (b) comparing the interferenceimage with a baseline interference image to produce a shearogram image,(c) repeating steps (a) and (b) at varying stress levels to produce aplurality of shearogram images, and (d) displaying the plurality ofshearogram images at a frame rate fast enough to generate an animationrepresentative of dynamically changing stress states on the test object.9. The method according to claim 8 further including the step of storingthe shearogram images.
 10. The method according to claim 8 wherein theframe rate is at least fifteen frames per second.
 11. The methodaccording to claim 8 further comprising simultaneously displayingmultiple shearogram image sequences representative of different sectionsof the test object.